In instruments which use extremely time stable magnetic field such as in NMR (nuclear magnetic resonance analysis) apparatuses or medical MRI (magnetic resonance imaging) apparatuses, an operation mode called a persistent current mode is desirable. With regard to the persistent current mode, a predetermined current is applied to a superconductive coil with an outside power supply to thereby generate a magnetic field, the positive and negative electrodes of the superconductive coil are then short-circuited at zero resistance, and the current of the outside power supply is made zero to thereby obtain a persistent current mode. Since the superconductive coil becomes a closed circuit of zero resistance, the superconductive magnet can continue to generate a predetermined magnetic field while the current does not decay. In this case, a device for short-circuiting the positive and negative electrodes of the superconductive coil is called a persistent current switch. The following properties are required for the persistent current switch.
(1) A current equivalent to the current which is applied to the superconductive coil can be applied at zero resistance.
(2) A superconductive state (on state) and a normal conductive state (off state) can be switched to one another by outside control, e.g. by heater heating.
(3) The resistance is high in the off state. For example, the resistance may be about several ohms.
On the other hand, superconductive magnets have heretofore been used by immersing them in liquid helium. However, because a high-temperature superconductor was discovered in the 1980's, and technologies for refrigerating machines have been developed, superconductive magnets have been able to be operated in a conductive cooling mode. The conductive cooling mode is a method for cooling a superconductive magnet by thermal conduction with a refrigerating machine. However, since a copper-oxide superconductor, which is the most typical of high-temperature superconductors, causes large temporal variations in a center magnetic field by magnetic flux creep, basically, a copper-oxide superconductor has not been operated in a persistent current mode.
In 2001, a new high-temperature superconductor of magnesium diboride was discovered. The critical temperature is 39 K, and, although the critical temperature is lower than that of a copper-oxide superconductor, the problem of the magnetic flux creep is small. Therefore, magnesium diboride is expected to be applied to a superconductive magnet which can be operated by a persistent current mode in a conductive cooling mode.